The General German Workers' Association (Allgemeiner Deutscher Arbeiterverein, ADAV) founded in 1863 and the Social Democratic Workers' Party (Sozialdemokratische Arbeiterpartei Deutschlands, SDAP) founded in 1869 later merged in 1875 under the name Socialist Workers' Party of Germany (Sozialistische Arbeiterpartei Deutschlands, SAPD). From 1878 to 1890, any grouping or meeting that aimed at spreading socialist principles was banned under the Anti-Socialist Laws, but the party still gained support in elections. In 1890, when the ban was lifted and it could again present electoral lists the party adopted its current name. In the years leading up to World War I, the party remained ideologically radical in official principle, although many party officials tended to be moderate in everyday politics. By 1912, the party claimed the most votes of any German party.

The SPD was established as a Marxist party in 1875. However, the Social Democrats underwent a major shift in policies reflected in the differences between the Heidelberg Program of 1925 which "called for the transformation of the capitalist system of private ownership of the means of production to social ownership"[11] and the Godesberg Program of 1959 which aimed to broaden its voter base and move its political position toward the centre.[12] After World War II, under the leadership of Kurt Schumacher the SPD re-established itself as a socialist party representing the interests of the working class and the trade unions. However, with the Godesberg Program the party evolved from a socialist working-class party to a modern social-democratic party working within liberal capitalism.

The current party platform of the SPD espouses the goal of social democracy, which is seen as a vision of a societal arrangement in which freedom and social justice are paramount. According to the party platform, freedom, justice and social solidarity form the basis of social democracy. The coordinated social market economy should be strengthened and its output should be distributed fairly. The party sees that economic system as necessary in order to ensure the affluence of the entire population. The SPD also tries to protect the society's poor with a welfare state. Concurrently, it advocates a sustainable fiscal policy that does not place a burden on future generations while eradicating budget deficits. In social policy, the Social Democrats stand for civil and political rights in an open society. In foreign policy, the party aims at ensuring global peace by balancing global interests with democratic means, thus European integration is one of the main priorities of the party. The SPD supports economic regulations to limit potential losses for banks and people. They support a common European economic and financial policy and to prevent speculative bubbles as well as environmentally sustainable growth.[13]

The SPD is mostly composed of members belonging to either of the two main wings, namely the Keynesian social democrats and Third Way moderate social democrats belonging to the Seeheimer Kreis. While the more moderate Seeheimer Kreis generally support the Agenda 2010 programs introduced by Chancellor Gerhard Schröder, the Keynesian social democrats continue to defend classical left-wing policies and the welfare state. The classical left-wing of the SPD claims that in recent years the welfare state has been curtailed through reform programs such as the Agenda 2010, Hartz IV and the more economic liberal stance of the SPD, which were endorsed by centrist social democrats.[citation needed] As a reaction to the Agenda 2010, there was in 2005 the ascension of an inner party dissident movement which led ultimately to the foundation of the new party Labour and Social Justice – The Electoral Alternative (Arbeit & soziale Gerechtigkeit – Die Wahlalternative, WASG). The WASG was later merged into The Left (Die Linke) in 2007.[14]

Before World War II, as the main non-revolutionary left-wing party the Social Democrats fared best among non-Catholic workers as well as intellectuals favouring social progressive causes and increased economic equality. Led by Kurt Schumacher after World War II, the SPD initially opposed both the social market economy and Konrad Adenauer's drive towards Western integration fiercely, but after Schumacher's death it accepted the social market economy and Germany's position in the Western alliance in order to appeal to a broader range of voters. It still remains associated with the economic causes of unionised employees and working class voters. In the 1990s, the left and moderate wings of the party drifted apart, culminating in a secession of a significant number of party members which later joined the socialist party WASG, which later merged into The Left (Die Linke).

Geographically, much of the SPD's current-day support comes from large cities, especially of northern and western Germany and Berlin. The metropolitan area of the Ruhr Area, where coal mining and steel production were once the biggest sources of revenues, have provided a significant base for the SPD in the 20th century. In the Free Hanseatic City of Bremen, the SPD has governed without interruption since 1949. In southern Germany, the SPD typically garners less support except in the largest cities. At the 2009 federal election, the party lost its only constituency in the entire state of Bavaria (in Munich). Small town and rural support comes especially from the traditionally Protestant areas of northern Germany and Brandenburg (with notable exceptions such as Western Pomerania where CDU leader Angela Merkel was re-elected in 2005) and a number of university towns. A striking example of the general pattern is the traditionally Catholic Emsland, where the Social Democrats generally gain a low percentage of votes, whereas the Reformed Protestant region of East Frisia directly to the north, with its strong traditional streak of Anti-Catholicism, is one of their strongest constituencies. Further south, the SPD also enjoys solid support in northern Hesse (Hans Eichel was mayor of Kassel, then Hesse's Minister-President and finally Finance Minister in the Schröder administration while Brigitte Zypries served as Justice Minister), parts of Palatinate (Kurt Beck was party leader until 7 September 2008) and the Saarland (political home of one-time candidate for federal chancellor Oskar Lafontaine, defected from the SPD in 2005).

The SPD, at times called SAPD, participated in general elections determining the members of parliament. For the elections until 1933, the parliament was called Reichstag, except of the one of 1919 which was called the National Assembly and after 1949 when it was called Bundestag. Note that changes in borders (1871, 1919, 1920, 1949, 1957 and 1990) varied the number of eligible voters whereas electoral laws also changed the ballot system (only constituencies until 1912, only party lists until 1949 and mixed system thereafter), the suffrage (women vote since 1919), the number of seats (fixed or flexible) and the length of the legislative period (three or four years). The list begins after the SPD was formed in 1875, when labour parties unified to only form the SPD (then SAPD, current name since 1890).

Maxwell, John Allen. "Social Democracy in a Divided Germany: Kurt Schumacher and the German Question, 1945-1952." Ph.D dissertation, West Virginia University, Department of History, Morgantown, West Virginia, 1969.

McAdams, A. James. "Germany Divided: From the Wall to Reunification." Princeton University Press, 1992 and 1993.

Erich Matthias, The Downfall of the Old Social Democratic Party in 1933 pages 51–105 from Republic to Reich The Making of the Nazi Revolution Ten Essays edited by Hajo Holborn, (New York: Pantheon Books, 1972).